The proposed studies will define precisely the anatomical relationships between steroid hormone target cells and catecholamingergic systems (noradrenaline, dopamine), as well as peptidergic systems in the central nervous system of cyclic female rates. These relationships will be studied by combined techniques of autoradiogrphy and immunohistorchemistry. Tritium labeled R5020, a synthetic progrestin, will be localized by authoradiography. Neuropeptides (enkephalin and neurotensin), neurohormones (luteinizing hormone releasing hormone, somatotropin releasing inhibiting factor) and catecholamine synthesizing enzymes (tyrosine hydroxylase, dopamine-Beta-hydroxylase) will be localized by the immunoperoxidase method. Since estradiol priming enhances progrestin binding in neural tissues, localization of 3H estradiol in tyrosine hydroxylase producting and dopamine-Beta-hydroxylase producing cells will be studied and the results will be compared with those of 3H R5020. Neurons containing both estrogen and progestin receptors will be identified using estradiol receptor antibodies for immunostaining and 3H R5020 for nuclear uptake of radioactivity. A dual immunoperoxidase staining technique will be applied in order to demonstrate topographic relationships between luteinizing hormone releasing hormone neurons and catecholamine system(s). Interaction of progrestin with brain catecholaminergic and peptidergic systems is important for the understanding of progrestin action on gonadotropin release and behavioral responses.